The present invention generally relates to the field of polishing. In particular, the present invention is directed to a CMP pad having a streamlined windowpane.
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited onto and etched from a surface of a semiconductor wafer. Thin layers of these materials may be deposited using any of a number of deposition techniques. Deposition techniques common in modern wafer processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) and electrochemical plating. Common etching techniques include wet and dry isotropic and anisotropic etching, among others.
As layers of materials are sequentially deposited and etched, the uppermost surface of the wafer becomes non-planar. Because subsequent semiconductor processing (e.g., photolithography) requires the wafer to have a flat surface, the wafer needs to be planarized. Planarization is useful for removing undesired surface topography as well as surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches and contaminated layers or materials.
Chemical mechanical planarization, or chemical mechanical polishing (CMP), is a common technique used to planarize workpieces, such as semiconductor wafers. In conventional CMP using a dual-axis rotary polisher, a wafer carrier, or polishing head, is mounted on a carrier assembly. The polishing head holds the wafer and positions it in contact with a polishing layer of a polishing pad within the polisher. The polishing pad has a diameter greater than twice the diameter of the wafer being planarized. During polishing, each of the polishing pad and wafer is rotated about its respective center while the wafer is engaged with the polishing layer. The rotational axis of the wafer is offset relative to the rotational axis of the polishing pad by a distance greater than the radius of the wafer such that the rotation of the pad sweeps out a ring-shaped “wafer track” on the polishing layer of the pad. When the only movement of the wafer is rotational, the width of the wafer track is equal to the diameter of the wafer. However, in some dual-axis polishers, the wafer is oscillated in a plane perpendicular to its axis of rotation. In this case, the width of the wafer track is wider than the diameter of the wafer by an amount that accounts for the displacement due to the oscillation. The carrier assembly provides a controllable pressure between the wafer and polishing pad. During polishing, a polishing medium is flowed onto the polishing pad and into the gap between the wafer and polishing layer. The wafer surface is polished and made planar by chemical and mechanical action of the polishing layer and polishing medium on the surface.
An important aspect of CMP is determining when polishing should be stopped, i.e., when the polishing endpoint has been reached. Generally, polishing is stopped either when a desired surface profile, or degree of planarization, has been achieved or when a desired thickness of a layer has been removed. One method of detecting the endpoint of polishing is to identify when a desired layer has been polished off the wafer using optical techniques. One example of such optical techniques is described in U.S. Pat. No. 5,433,651 to Lustig et al. Generally, these optical endpoint detection techniques involve reflecting a light beam, e.g., laser beam, off of the wafer being polished, measuring the reflected light, and determining when the reflectance changes. A relatively abrupt change in reflectance often occurs when a layer having a first reflectance has just been polished away to expose another layer having a second reflectance different from the first reflectance.
Since CMP pads are typically opaque, CMP pads used in connection with optical measuring systems are often provided with various shaped translucent or semi-translucent windowpanes that allow a light beam to strike and reflect off of the wafer without moving the wafer away from the pad. The most common CMP pad windowpane shapes are blunt shapes, such as rectangular, circular and shapes having aspects of both circular and rectangular shapes. For example, U.S. Pat. No. 6,458,014 to Ishikawa et al. discloses a CMP pad that includes a rectangular windowpane. U.S. Pat. No. 6,537,133 to Birang et al. discloses a CMP pad that includes a circular windowpane and a CMP pad that includes an elongate arc-shaped slotted windowpane having semi-circular leading and trailing ends.
FIGS. 1A and 1B illustrate how the polishing medium flow in the gap between a wafer 100 and a conventional CMP pad 104 is affected by a rectangular-shape windowpane 108. In this case, CMP pad 104 includes a polishing surface 112 having a plurality of concentric, circular grooves 116, and windowpane 108 is rectangular in shape, with its long axis 120 located along a radius 124 of the pad. Although polishing surface 112 contains grooves 116, it is typically not practical to put grooves in windowpane 108 because such grooves, or the polishing debris that would collect in them, may scatter a light beam (not shown) shone through the windowpane and, consequently, confound the signal reaching an endpoint detector (not shown).
As clearly shown in FIG. 1B, the approaching polishing medium flow (as indicated by flow lines 128) within grooves 116 confronts a “leading” long side 132 of windowpane 108 and is essentially deflected around the windowpane. In addition to the polishing medium essentially backing up against windowpane 108 along leading long side 132, the polishing medium flow adjacent the short sides 136 of the windowpane is increased by the additional amount of polishing medium that would have flowed through the region at the windowpane had the window not been present. Finally, the flow of polishing medium immediately adjacent the trailing long side 140 of the window is greatly disturbed by the blockage created by the window because flow gathers inward behind the window from both short sides 136 and converges in a disorderly manner along trailing long side 140. Needless to say, the polishing medium flow in the entire region surrounding windowpane 108 is greatly disturbed by the presence of the windowpane. Although a small amount of polishing medium may traverse the top surface of the window in a very thin layer, the other disturbances to the flow are not reduced.
Generally, the greater the obstruction to the polishing medium flow resulting from the presence of a windowpane, such as windowpane 108, the greater the probability that the resulting flow disturbances will have a negative impact on the polishing process. This is so because the disturbed flow thwarts an even distribution of polishing medium chemistry and uniform temperature field, contributing to non-uniformity in point-to-point polishing rates across the wafer. In addition, the termination of many grooves at the edge of a blunt leading edge of a windowpane provides an opportunity for polish debris to accumulate, potentially leading to scratches and other defects.
None of the patents mentioned above, nor the designers of conventional CMP pad windowpanes appear to give much, if any, consideration to the effect of the plan-view shape of the windowpane on polishing nor the impact of the windowpane on polishing medium flow patterns in the pad-wafer gap, with the exception of flushness of the windowpane to the surrounding polishing surface. Consequently, what is needed is a polishing pad that has a windowpane and is designed to reduce the impact of the windowpane on polishing and on the disruption of polishing medium flow within the pad-wafer gap.